In a networked environment such as the Internet, entities such as content providers may provide information for display on a resource (e.g., a web page). The resources may include text, video, graphics, or audio information provided by the entities via a server. Additional or alternative content items may also be provided by third parties for concurrent display with the main content. Thus, a user device accessing a resource may be provided with the main content of the resource, as well as third party content items. A content distribution system may be used to determine which third party content to provide for concurrent display.